Module 1 - situation: body ambient bondgraph model using heat flux  (Page 3/3)

Environmental conditions can extreme in occupations such as firefighters or other workers in high heat environments. Protective equipment is designed as a thermal insulator, which also serves to prevent the body from dissipating heat.

Current practices for protection against heat stress

The current approach to protection against thermal stress involves worker education. Employees are given information on how antagonistic conditions exacerbate overheating risks and how to recognize the early signs of heat stress illness.

Figure 3: Sample Educational Chart for Self Monitoring Against Heat Stress Illness

The Wet Bulb Globe Temperature (WBGT) index is considered the authoritative standard in providing employees usable information in quantifying the severity of antagonistic conditions. It is derived by taking measurements with three devices that separate heat flow components all effects save for wind speed, all effects, and ambient temperature. These measurements are then combined by means of a weighted average to the WBGT.

Unfortunately, while the WGBT method is accurate and reliable, it is also requires expensive equipment with high maintenance requirements. As such this method is impractical for small scale facilities, and such data is not even recorded at typical meteorological sites.

Figure 4: Dry and Wet Bulb Measurement Setup for WBGT Index Data

Alternate practices

There are two established methods of reliably monitoring body core temperatures for a mobile person. These include probes inserted within the esophagus, and alternately the rectum. Both are uncomfortable, and impractical for periods in excess of 24 hours.

A third device still under development and involves a wireless device which is ingested, providing persistent monitoring as it works it’s way through the digestive system over a period of 18 to 36 hours. Studies to date show the performance to be nearly as dependable as the established esophageal and rectal thermometers. Its primary drawbacks are that the devices are not reusable, and has not yet been widely accepted by the market.

Various devices have been developed which monitor skin temperature. Skin temperature itself is a poor estimate of core temperature due to the greatly variable impedances between core to skin, as well as skin to ambient. Some efforts couple skin temperature with other measured factors, such as heart rate, accelerometer data, and WBGT index information in an effort to arrive at a more accurate estimation. None of these efforts have proceeded past initial trials.

Bondgraph model and heat flux transducer

This paper proposes a bondgraph model for the understanding of the heat transfer mechanisms between the human body and ambient under various conditions. Individual contributions to body heat are shown as r-elements. A measurement device of a heat flux transducer is shown as a transformer element. The equation layer of the model can be tailored for various operating conditions, using either derived or empirical formulas to describe heat transfer.

Figure 5: Bond Graph Model of Heat Flow and Temperature of the Human Body

The model shows the same components of body-ambient heat transfer as are found in most occupation educational literature. The model further demonstrates causality and direction of power flow. The system equation layer provides the ability to mathematically quantify contributing elements of system heat transfer.

Results

This approach holds advantages over existing methods in that the person at risk is no longer responsible for their own protection, thereby removing certain conflicting interests in making a determination. The model presents total system data for the body and ambient environment.